An alternative technology using cryogenic silicon test masses and a 2 laser [1]} has been identified as a potential upgrade to the baseline Cosmic Explorer technology of room-temperature fused silica test masses and a 1 laser and could also be used should thermal effects in the baseline technology prove intractable [2]}. There are several new considerations with this technology [3]}. First, the light traversing the substrates of the test masses experiences a phase noise due to the temperature dependence of the index of refraction which is a potentially significant low frequency noise source for the silicon technology, especially for the 20 detector, due to silicon's larger thermorefractive coefficient and thermal conductivity. This thermorefractive noise is suppressed by a factor of \(\sqrt{\mathcal {F}}\) , however, which presents a trade off between low frequency sensitivity favoring large \(\mathcal {F}\) , and high frequency sensitivity favoring small \(\mathcal {F}\)  to minimize SEC loss. Second, the need to radiatively cool the cryogenic test masses imposes a strict heat budget [1]} which adds an additional constraint on how low \(\mathcal {F}\)  can be made to limit the power absorbed in the optics. This makes developing low absorption and high quality silicon substrates and optical coatings for 2 light particularly important. Finally, manufacturing high quantum efficiency photodiodes for 2 light is a critical area of R\(\&\) D necessary to minimize readout loss (c.f. fig:20kmpmquantum) for this technology.
